The zone-wide encounter starts with a new monstrosity called the Seed of Hamidon, which drops seedlings throughout the Sunken City neighborhood of First Ward. These seedlings begin the spread of the Tellurian Plague. It is in the best interest of heroes, villains, loyalists, and resistance rebels alike to end the spread of this plague before it conquers everything, as it did on Praetorian Earth.
Arbiter Hawk suggests an eight-person group for this encounter, which takes place in two distinct phases. In the first phase, it's up to players to take out both the ground and air seedlings. Don't worry, non-flying characters, because as these ground-based seedlings are destroyed, they grant Buoyant Membranes that allow players and nearby allies to take flight. The Seed weakens as players defeat the seedlings until the Seed is forced to extinguish the remaining seedlings in order to regenerate power. This is when you attack and the second phase begins. Take out the Seed quickly before it is able to completely recharge -- or the cycle starts over again.
The dev diary contains many of the deadly details of the seedlings as well as more advice from Arbiter Hawk; be sure to read the entire thing before battling this globular fiend.